The invention relates to a data receiver comprising an adaptive system for estimating at least one measure of a characteristic quantity of an input signal applied to an input of the data receiver, which adaptive system includes:
error determining means for determining an error in the estimate of the measure of the characteristic quantity of the input signal; PA1 adapting means for adapting at discrete instants the estimate of the measure of the characteristic quantity of the input signal; PA1 the input of the data receiver is coupled to a first input of the error determining means; PA1 for providing the error an error output of the error determining means is coupled to an input of the adapting means for periodically conveying the error with a period nT, where n is an integer and T a rational fraction (.ltoreq.1) of an interval between received symbols; PA1 at least one output of the adapting means, the output having an output signal which forms a measure of the characteristic quantity of the input signal, is coupled to a second input of the error determining means; PA1 the adapting means adapt their output signal at discrete instants in order to minimize the error.
in which
An adaptive system of this type is known from European Patent Specification 0 065 796.
When signals are transmitted by way of a transmission medium, for example, a subscriber line of the public telephone network, or stored on a recording medium, for example, a magnetic tape or disc, it is generally necessary when these signals are received or reproduced respectively, to process the received signals before the desired output signals are present.
The necessary signal processing is usually determined by the properties of the transmission or recording medium respectively, such as, for example, the frequency characteristic, by the properties of the signal itself, such as, for example, the spectral power density, or by the properties of any occurring interfering signals, such as, noise or cross talk.
If said properties of the medium, of the signal or of the interfering signals are known in advance, and do not change with time, the signal processing necessary for obtaining the desired output signal may be performed in a standard manner.
Usually, however, these properties will not be known in advance or will change with time. The use of a system that performs the necessary signal processing in a standard manner, will generally not lead to an optimum recovery of the desired output signal.
In order to avoid this, adaptive systems are often used which are capable of adapting the signal processing to the properties of the transmission channel, to the properties of the transmitted signals or to the interfering signals.
Examples of suchlike systems are adaptive equalizers, adaptive echo cancellers and clock recovery systems. These systems comprise an error determining unit which derives an error from both the input signal and the estimate of a characteristic quantity of the input signal, which error is a measure for the deviation of the estimated characteristic quantity of the input signal relative to the real characteristic quantity of the input signal. A characteristic quantity may be, for example, the intersymbol interference, cross talk or echoes present in the input signal.
If the characteristic quantity of the input signal relates to properties of an interfering signal present in that input signal (for example, echoes or intersymbol interference), this interfering signal may be cancelled with the aid of a signal processing unit controllable by means of the estimate of the characteristic quantity, so that an output signal may be obtained which is substantially free from that interfering signal.
In the receiver according to the state of the art as shown in FIG. 1, the characteristic quantity to be estimated of the input signal is an echo of the signal transmitted by the station concerned and present in this input signal. This echo signal is caused by cross talk of the hybrid circuit B and reflection of the transmitted signal in the remote-end hybrid circuit. This echo signal is represented by the parameter values of the filter E in the adaptive system which values are necessary for obtaining a replica of this echo signal. C indicates a transmission path, and arrows along this path denote a unidirectional send and receive path respectively.
In the data receiver comprising the echo canceller according to the state of the art as shown in FIG. 1, the error determining unit comprises the digital filter E, the D/A converter F, the filter G, the subtractor circuit H, the sample-and-hold circuit S and the section of the circuit Q in which the error is determined on the basis of the input signals of the circuit Q. On the basis of the error r(t) the estimate of the characteristic quantity of the input signal may now be adapted for error minimization. In the known data receiver this adaptation is performed by the adapting means Q adapting the parameters of the filter E on the basis of the signal r(i) derived from the error r(t) by means of the A/D converter N and switch P.
Generally, the error is determined periodically with a period T, and a desired adaptation of parameters of the signal processing unit is made. Thus, every T seconds an adaptation of these parameters is made.
The receiver according to the state of the art further comprises a multiplier XM with a multiplying factor M, and a low-pass filter D. Symbols and signals are indicated by a(k) and b(k), and e(t), r(t) and s(t) respectively. A clock signal is indicated by 1/T.sub.s.
The block 10 represents a data signal source producing data symbols a(k), wherein k represents the number of the data signal, which data symbols occur at a rate of 1/Ts symbols/sec., wherein Ts represents the interval in sec. between data symbols.
In order to restrict the complexity of suchlike adaptive systems, the estimate of the characteristic quantity is not adapted periodically with the period T in the receiver known from afore-mentioned Patent Specification, but this is effected with a period nT on the basis of the error periodically determined with a period nT. This is often permissible because the characteristic quantity of the input signal often shows only slow changes with time. In the data receiver according to the state of the art the sample-and-hold circuit S takes a sample of the difference signal not more than once per nT seconds. At lower transmission rates this value of n may be, for example, 10, but at extremely high transmission rates n may, for example, exceed 1000.
Experiments have shown that convergence to a minimum difference between the estimated characteristic quantity of the input signal and the real characteristic quantity of the input signal fails to occur in several adaptive systems according to the preamble.